There is no difference between creating an account and signing in an existing account.
The first time you give your email address on the authentication page, your account is created and you receive an email with link to sign in. The next times you'll have to sign in, you will proceed the same way − but obviously no new account will be created.
More information about signin into Skriv: How to sign in Skriv
The free trial period begins when you create your account.